1. Field of the Invention
The present invention relates to a scintillation camera, more particularly to the scanning of a subject by means of a detector incorporated in the scintillation camera, and to a non-contact type sensor for use in a nuclear medicine diagnostic apparatus or in an X-ray diagnostic apparatus.
2. Description of the Related Art
A scintillation camera is used to form a functional image of a living subject while a detector is scanning the subject and detecting the gamma-rays emitted from the radioisotope labeled with a specific medicine administered to the subject.
The camera serves to determine, from the gamma-rays detected, the distribution of the radioisotope within the subject, in order to thereby visualize the shape and functional condition of an organ, the presence or absence of a lesion, and metabolic function of the subject. Although it has a low resolution and involves radiation exposure of the subject, it is used in specific fields such as the early detection of a brain ischemia section and estimation of the viability of myocardial cells. The scintillation camera serves as an apparatus which assists an X-ray computed tomography apparatus.
A conventional scintillation camera is operated to scan to obtain a SPECT (Single Photon Emission Computed Tomographic) image of a subject by one of the following two alternative methods.
The first method, generally known as "4-point determination method," serves to input four points before the detector of the camera is moved around a subject to scan the subject. Of these points, two are on a major axis extending in the width direction of the subject, and the other two are on a minor axis extending in the thickness direction of the subject. That is, an appropriate distance from the center of rotation along the turning radius is input every time the detector is rotated by 90.degree., thereby calculating an arc for each quadrant. The detector is thereby moved in an elliptical orbit passing the four points.
The second method is to rotate the detector once around the subject in an elliptical orbit, before the acquisition of data from the subject, thereby obtaining data representing the elliptical orbit. Then, the detector is moved around the subject along the elliptical orbit represented by the data, to scan the subject.
In the first method, data items representing the four points must be input to define an elliptical orbit, prior to the scanning of the subject. In the second method, the detector must be moved once around the subject in an elliptical orbit to acquire the data representing this orbit, prior to the scanning of the subject. In either method it is necessary to set an orbit for the detector before the acquisition of data from the subject. Furthermore, the subject may move after the orbit has been set, making it necessary to adjust the orbit. However, the orbit can hardly be adjusted once it has been thus set or once the data acquisition has been started.
Particularly, in the 4-point determination method, the orbit for the detector is an elliptical one which has not been set on the basis of the outline of a transverse section of the subject. The orbit is not an ideal orbit which should preferably closely surround the subject.
As indicated above, the conventional scintillation camera cannot be used before an orbit is set for the detector or the data representing such an orbit is input. The subject may move, making it necessary to adjust the orbit. However, it is difficult to adjust the orbit once the orbit has been set or once the data acquisition has been initiated. Further, in the 4-point determination method it is impossible to set an ideal orbit closely surrounding the subject.
A scintillation camera of so-called "whole body type," is known, which scans a subject while its detector is moving along the body axis of the subject. The conventional whole body-type scintillation camera is operated to scan a subject by one of the following three alternative methods.
In the first method, as shown in FIG. 1A, the detector 48 is located at a position which has been determined based on the highest point of the ridge line showing the lateral section of the subject 50 lying on the bed (hereinafter, referred to as only a subject for simplicity in some cases). Then, the detector 48 is moved along the body axis of the subject 50, while maintained at that position, to thereby scan the whole body of the subject 50.
In the second method, as shown in FIG. 1B, the detector 48 is moved along the ridge line of the subject 50, prior to the scanning of the subject 50, thereby obtaining data representing the curved path of the detector 48. Then, the detector 48 is automatically moved along the curved path represented by the data thus obtained, to thereby scan the whole body of the subject 50.
In the third method, the detector is located at a scanning start position, a scanning length, or a scanning end position, is determined on the basis of the height of a subject and input, and the detector is moved from the scanning start position to the scanning end position. While being so moved, the detector acquires whole-body data from the subject.
The three methods have problems, however.
The first method is problematic in two respects. First, it takes many times to determine a proper position for the detector 48. Second, the detector 48 is positioned too far a distance away from the subject 50, except for a moment it is located close to the highest point on the ridge line. The resolution of the scintillation camera is therefore decreased.
The second method is problematic in two respects, too. First, it takes too much time to obtain the data showing the curving path of the detector 48. Second, data acquisition from the subject 50 is interrupted when the subject 50 moves, inadvertently touching the detector 48, as often happens, while the detector 48 is moving along the curving path.
The third method is problematic in two respects, too. First, an operator must take pains to locate the detector at a scanning start position where the head or toes of the subject is completely within the view field of the detector, while looking at the image picked up by the detector. Inevitably it takes a long time to locate the detector at a desirable scanning start position. Second, it is necessary for the operator to determine the scanning length, or the scanning end position, on the basis the height of the subject, and to input the scanning length. To determine and input the scanning length is also time-consuming.
Scintillation cameras are used, each in combination with one sensor.
The sensor comprises a plurality of light-emitting elements and a plurality of light-receiving elements, which are arranged in the same plane. The light-receiving elements oppose the light-emitting elements, respectively, spaced apart therefrom by a predetermined distance. The sensor is designed for industrial use. When all light-receiving elements receive light from the associated light-emitting elements, it is determined that no object lies between the light-emitting elements on the one hand, and the light-receiving elements on the other hand. When any one of the light-receiving elements does not receive light from the associated light-emitting element, it is determined that an object lies between the light-emitting elements and the light-receiving elements.
FIG. 2 schematically shows a conventional sensor of this type.
As shown in FIG. 2, a plurality of light-emitting elements 11.sub.1 to 11.sub.N and a plurality of the light-receiving elements 12.sub.1 to 12.sub.N are arranged in the same plane. The elements 12.sub.1 to 12.sub.N oppose the light-emitting elements 11.sub.1 to 11.sub.N, respectively, spaced apart therefrom by a predetermined distance. Each light-receiving element 12 receives the light emitted from the associated light-emitting element 11. The optical axis extending between each light-emitting element 11 and the associated light-receiving element 12, shown by a broken line, is parallel to the optical axis extending between any other associated elements 11 and 12. When an object 50 exists between the light-emitting elements 11 on the one hand and the light-receiving elements 12 on the other hand, at least one of the light-receiving elements 12 cannot receive the light emitted by the associated light-emitting element 11. Thus, the presence or absence of an object can be determined from the signals output by the light-receiving elements 12. The light-emitting elements 11 and the light-receiving elements 12 are controlled by a sensor controller 20. The controller 20 generates a signal representing the presence or absence of an object. This signal is used for various purposes. The plane in which the elements 11 and 12 are arranged is either horizontal or vertical, in accordance with the purpose for which the sensor is employed.
The sensor for industrial use, shown in FIGS. 3A and 3B, may make errors when a light-reflecting object is located in the vicinity of the light-emitting elements 11 and the light-receiving elements 12.
As shown in FIG. 3A, a light-reflecting flat object may be located near the light-emitting elements 11 and the light-receiving elements 12. In this case, the light emitted from any light-emitting element 11 is reflected or scattered on the reflecting surface of the object and is then applied to the light-receiving element 12 associated with the light-emitting element 11.
As shown in FIG. 3B, the sensor may be located near a light-reflecting wall or floor. In this case, too, the light emitted from any light-emiting element 11 is reflected or scattered on the reflecting surface of the object and is then applied to the light-receiving element 12, associated with the light-emitting element 11.
In either case, the sensor outputs a signal representing the absence of an object, despite the fact that there is an object located between the light-emitting element 11 and the light-receiving element 12.
A sensor of the type shown in FIG. 2 is often used in combination with the medical scintillation camera described above, with its light-emitting elements 11 attached to one end of the camera, and its light-receiving elements 12 secured to the other end of the camera. The scintillation camera, which is a reflector, extends parallel to the optical axis of the sensor. The light reflected from and scattered by the scintillation camera adversely affects the reliability of the sensor. When the sensor makes an error due to the reflected or scattered light, the camera may abut on a subject or may be stopped.